Conventionally, a semiconductor device having an airtight chamber between a first substrate and a second substrate described below has been proposed (see, e.g., Patent Literature 1). In this semiconductor device, a sensing portion which detects an angle velocity is formed on the first substrate. In the second substrate, a recessed portion is formed on one surface facing the sensing portion formed on one surface of the first substrate. The second substrate is joined to the first substrate to provide the airtight chamber which seals the sensing portion in a room defined by a space between the first substrate and the recessed portion. The airtight chamber is under a vacuum pressure.
The manufacturing method of the above-described semiconductor device is described below. First, the sensing portion detecting the angle velocity is provided to the first substrate, and the recessed portion is formed in the second substrate. Subsequently, the first substrate and the second substrate are joined together to configure the airtight chamber which seals the sensing portion in the room defined by the space between the first substrate and the recessed portion.
In a joining of the first substrate and the second substrate, it is known that OH groups are formed on respective joining surfaces of the first substrate and the second substrate, and a strength of joining can be enhanced by a covalent bond of OH groups formed on respective joining surfaces. Though the above-described method using chemical bond can enhance the strength of joining between the first substrate and the second substrate, there is a difficulty that a pressure in the airtight chamber becomes higher than an intended pressure due to hydrogen gas (that is, degassing) which is generated in the airtight chamber from the OH groups.
In view of the foregoing difficulties, it is considered that a diffusion length (that is, diffusion coefficient) of the hydrogen gas in the airtight chamber may be increased by a heat treatment so that the hydrogen gas is diffused into (that is, passed through) the first substrate or the second substrate and emitted to an outside. When the heat treatment is not performed adequately, the pressure in the airtight chamber may fail to reach an intended pressure. That is, the pressure in the airtight chamber may be fluctuated.
The above-described difficulty does not arise only in the airtight chamber sealing the sensing portion. A similar difficulty also arises, for example, in a pressure sensor or the like having the airtight chamber as a reference pressure chamber when the airtight chamber is provided by joining the first substrate and the second substrate together.